Microporous polymeric membranes are well known in the art and are used widely for filtration and purification processes, such as filtration of waste water, preparation of ultrapure water and in medical, pharmaceutical or food applications, including removal of microorganisms, dialyses and protein filtration. While these membranes have found broad utility for a variety of purposes, they suffer from several disadvantages, such as broad and frequently non uniform pore size distribution, and limited chemical, solvent and thermal resistance.
Poly(aryl ether ketone)s represent a class of semi-crystalline engineering thermal plastics with outstanding thermal properties and chemical resistance. One of the representative polymers in this class is poly(ether ether ketone), PEEK, which has a reported continuous service temperature of approximately 250° C. PAEK polymers are virtually insoluble in all common solvents at room temperature. These properties make PAEK attractive materials for porous membrane preparation. However, the application of PAEK polymers in membrane fabrication has been limited owing to their intractability, which prevents the use of conventional solvent-based methods of membrane casting.
It is known in the art to prepare porous PEEK membranes from solutions of strong acids, such as concentrated sulfuric acid. PEEK can undergo sulfonation in the concentrated sulfuric acid media and thus lose some of its desirable sought after properties. U.S. Pat. No. 6,017,455 discloses preparation of non-sulfonated porous PEEK membranes from concentrated sulfuric acid solvents sufficiently diluted by water to prevent sulfonation. The membranes are formed by casting PEEK solution to form a film followed by coagulation in a concentrated sulfuric acid. This membrane preparation process is highly complicated and produces large amounts of waste acid.
U.S. Pat. No. 5,997,741 discloses preparation of porous PEEK membranes by forming a solution of PEEK polymer in a concentrated sulfuric acid at the temperature of 15° C. or lower to prevent sulfonation. The solution is processed and cast at a subambient temperature, followed by coagulation in water or in a concentrated sulfuric acid. Only dilute PEEK solutions can be formed in the concentrated sulfuric acid which adversely affects film forming characteristics, the mechanical characteristics, and the pore morphology of the porous PEEK membranes.
U.S. Pat. Nos. 4,992,485 and 5,089,192 disclose preparation of PEEK membranes from non-sulfonating acid solvents that include methane sulfonic acid and trifluoromethane sulfonic acid. European Patent Specification EP 0737506 discloses preparation of improved polymeric membranes based on PEEK admixtures with polyethylene terephthalate. The membranes are formed by the solution casting process from a methane sulfuric acid/sulfuric acid solvent mixture.
The acid based solvent systems for manufacturing of porous PEEK membranes disclosed in the art are highly corrosive, frequently toxic and generate substantial environmental and disposal problems. For these and other reasons, the acid based casting processes have found limited commercial use.
An alternative to the acid based solvent system for PEEK membrane preparation involves the use of high boiling point solvents and plasticizers that dissolve PEEK polymer at elevated temperatures. U.S. Pat. Nos. 4,957,817 and 5,064,580, both issued to Dow Chemical Co., disclose preparation of porous PEEK articles from its admixture with organic polar solvents having a boiling point in the range of 191° C. to 380° C., such as benzophenone and 1-chloronaphthalene, and organic plasticizers capable of dissolving at least 10 weight percent of PEEK, respectively. The final porous article is formed by removing the organic polar solvents and/or plasticizers by dissolution into a low boiling temperature solvent.
U.S. Pat. No. 5,200,078 discloses preparation of microporous PEEK membranes from its mixtures with plasticizers wherein the membrane undergoes a drawing step prior to or after the plasticizer is removed by leaching.
U.S. Pat. No. 5,227,101 issued to Dow Chemical Co. discloses preparation of microporous membranes from poly(aryl ether ketone) type polymer by forming a mixture of an unsulfonated PEEK type polymer, a low melting point crystallizable polymer, and a plasticizer, heating the resulting mixture, extruding or casting the mixture into a membrane, quenching or coagulating the membrane and leaching the pore forming components.
U.S. Pat. No. 5,205,968, issued to Dow Chemical Co., discloses preparation of microporous membranes from a blend containing a poly(aryl ether ketone) type polymer, an amorphous polymer and a solvent.
M. F. Sonnenschein in the article entitled “Hollow fiber microfiltration membranes from poly(ether ether ketone)”, published in the Journal of Applied Polymer Science, Volume 72, pages 175-181, 1999, describes preparation of PEEK hollow fiber membranes by thermal phase inversion process. The use of a leachable additive polymer, such as polysulfone, is proposed to enhance membrane performance.
It is also known in the art to prepare porous PEEK membranes from its blends with a compatible poly(ether imide) polymer, PEI. Preparation of such membranes is described by R. S. Dubrow and M. F. Froix in U.S. Pat. No. 4,721,732 and by R. H. Mehta et al. in an article entitled “Microporous membranes based on poly(ether ether ketone) via thermally induced phase separation”, published in the Journal of Membrane Science, Volume 107, pages 93-106, 1995. The porous structure of these PEEK membranes is formed by leaching the poly(ether imide) component with an appropriate strong solvent such as dimethylformamide. However, as described by Mehta et al., the quantitative removal of PEI component by leaching is essentially impossible (i.e., Mehta et al. are unable to achieve substantially complete removal of PEI) which in turn can lead to an inferior membrane performance.
Japan Kokai Tokkyo Koho 91273038 assigned to Sumitomo Electric Industries, Ltd., discloses preparation of porous PEEK membranes by an ion track etching method.
A recent report by Y. Ding and B. Bikson entitled “Novel macro and meso porous materials prepared from polysulfone/polyimide blends” in “Material Research Society Symposium Proceeding”, Volume 752, AAA1.3, Boston, 2002, describes preparation of porous polysulfone films from its blends with a phenylindane containing polyimide by decomposing the phenylindane containing polyimide with a hydrazine solution in methanol.
M. L. Bailey et al. in U.S. Pat. No. 5,651,931 describe a sintering process for the preparation of biocompatible filters, including PEEK filters. The filters are formed from a PEEK powder of a preselected average particle size by first pressing the powder into a “cake” followed by sintering in an oven or furnace. The process is limited to preparation of filters with a relatively large pore size and a broad pore size distribution and does not provide a economic means of forming large membrane area fluid separation devices.
Thus, as can be recognized from consideration of the prior art, when different polymers are intertwined, removal of one particular polymer so intertwined is extremely difficult in terms of substantial completeness (i.e., on the order of about 95% or greater) within the limited time frame necessary for efficient industrial manufacturing.
Thus there still remains a need in the art to produce porous PAEK articles, including porous membranes with improved properties, by a simple and commercially scalable process that is environmentally benign and does not utilize corrosive or toxic chemicals in membrane fabrication.